1. Field of the Invention
This invention relates to electronic circuits and, more particularly, to devices for generating noise with computer-controlled spectral shaping.
2. Description of the Prior Art
Men and other animals are endowed with the ability to hear sounds. Sounds are either produced by nature or by men and animals. Inanimate nature produces the thunder of the storm, the roar and pounding of the surf, the whistling of the wind, the whispering of the trees, the patter of rain, the rippling and gurgling of running water and the creaking of snow, etc. Even more rich is the sound of living things; the barking and snarling of dogs, roaring of lions, hissing of snakes, chirping of birds and crying of infants, etc.
Finally man, not content with the racket around him over which he has little or no control, has contrived to produce sounds of almost infinite variety. The boom of cannons, the whir of machinery, and the ticking of clocks, etc. Thus, man is immersed in a world of sound and he expects to hear a particular distinct sound upon the happening of certain events.
There are far more to sounds than meets the ear. In fact, sounds are caused by the vibration of some medium. For a complete description of sound, one must know the sound spectrum, i.e., the sound power or amplitude being carried by the various frequencies in the audible and inaudible range.
Man has invented simulators and other training devices to teach students how to use and operate various types of equipment without undergoing the dangers that are inherent in the equipments' actual operation. In order for the student to receive the maximum training benefit while he uses a simulator, the simulator should make the same noises as the actual equipment.
One of the methods employed by the prior art to dynamically change the noise source comprised an analog random noise generator, the output of which was filtered by a bank of one-third octave filters. The output of each filter was when then attenuated by a computer-controlled attenuator to provide the correct level for each filter frequency band. The attenuators' outputs were summed to produce the spectral shape of the noise. A large number of filters are required in order to obtain any significant degree of noise shaping. Thus, the above method is costly and requires a large amount of space.